1. Technical Field
The present invention relates to alloys based on cobalt and/or nickel and a method for the production thereof by a powder metallurgy route.
2. Related Art
Turbochargers for internal combustion engines have been in common use for many years in order to increase the power output and decrease the emissions of an engine. However, one drawback with the use of turbochargers has been the so-called “turbo lag” where the engine speed response to the throttle is delayed owing to the time needed for the exhaust gas turbine to increase speed and hence supply further air by the compressor for combustion. In recent years, however, turbocharger designs have been introduced which have largely overcome or at least greatly reduced the turbo lag problem.
Reduction in turbo lag has generally been achieved in one of two main ways. Firstly, there are designs which have moving vanes in order to alter the approach gas speed and direction on to the exhaust turbine to utilise the exhaust gas flow more efficiently at low turbine speeds and hence enable the compressor impeller wheel to accelerate more rapidly in response to throttle demand. Such designs are complex and hence very costly to manufacture. Secondly, there are designs which alter the volume of the exhaust gas passage in the turbocharger casing thus, influencing the exhaust gas velocity and again enabling the exhaust driven compressor to accelerate more rapidly in response to throttle demand.
Both types of design may be broadly described as variable geometry devices.
Turbochargers are used on both gasoline and diesel oil fueled vehicles. Generally, turbochargers for gasoline engines run at a higher temperature than do turbochargers for diesel oil fueled vehicles. Furthermore, due to the superior fuel efficiency of diesel engines, there is an increasing drive to employ turbocharged diesel engines in mass produced “family” type vehicles. Effectively, such diesel powered vehicles are intended to drive and behave in a manner substantially indistinguishable from gasoline fueled vehicles including the performance thereof.
As noted above, turbochargers in gasoline engines run at a higher temperature than those for diesel engines and furthermore, the general temperature levels for both types are rising as overall engine performance increases.
Unlubricated bearings and components in contact with the exhaust gas for use in turbochargers must be able to survive at 1050° C. without significant oxidation. Consequently, the alloys available for such applications tend to be relatively very expensive cobalt-based or nickel-based alloys made by investment casting or hot isostatic pressing (HIP) full densification with mechanical properties and surface degradation resistance more applicable to high-cost, critical components in aircraft gas turbine engines, for example. Components made by these production processes are very expensive and have performance parameters in excess of that required in, for example, bearings for turbocharger applications.
In the modern types of turbocharger described above, the unlubricated bearing or bush is primarily required to have good oxidation resistance at its operating temperature rather than high creep resistance, as required in rotating parts such as blades for gas turbine engines, for example. Furthermore, internal defects such as relatively high levels of porosity, prior particle boundaries and grain boundary precipitates may be allowable.
It is an object of the present invention to provide a material and method for the processing thereof suitable for use as an unlubricated bearing and/or bush in a turbocharger at an economic production cost.